delete or rename the wp-signup.php file or put in a die(); right at the beginning. The only other way new blogs would then be created is via the form at the bottom of the Dashboard -> Site Admin -> Blogs page

You can use the 'wpmu_activate_blog' action to deactivate a newly created blog. Look at line 1131 of wpmu-functions.php. I'd set the deleted field in wp_blogs for the new blog to '1', filter the welcome message using the 'update_welcome_email', and/or edit it through the site admin options page to inform the user that they're blog must be moderated. You'll have to build a page to moderate new signups too, but that shouldn't be too hard. It should of course send a welcome email to the new blog owner

Wordpress' important user functions and such are in the wp-includes/pluggable.php file and have been designed to be overridable; a plugin could replace those functions and implement its own user login and authentication scheme.

Best bet would be to use the Limit the email registrations feature at Dashboard -> Site Admin -> options and set it to only your companies email domain; remove the all of the links to the signup page as well even getting rid of the signup file

put a redirect in the top of wp-signup that redirects them to a "custom" form on your site that mimics the signup process; then, create a blog in the back end (or add a user) if you want to "approve" it

Avatars

WPMU Avatar widget users will be able to navigate to their Profile page and upload their own photo. The photo will appear next to their comments across your WPMU site, and, if they use my Author Profile Enhanced widget, in their sidebar next to their “About Me” text (demo)

Upload form fix (for IE7 only?) open up wp-admin/profiles.php, find the form line about 15 lines down and change it to this: <form name="profile" id="your-profile" action="profile-update.php" method="post" enctype="multipart/form-data">

Gravatar allows you to generate a gravatar URL complete with rating, size, default, and border options; it also shows LiveJournal userpics in comments

Localized comment avatar allows readers to specify a name, e-mail address and/or URL that they will always use when commenting, and to upload and assign an image to this combination of specified values

From Jenny: the font I used was ITC Franklin Gothic Book. Jason was wondering about the font being square. Any of the Franklin Gothic fonts will give you the "roundedness" that the theme has that I drafted.